Method of and apparatus for desulfurizing pig iron

ABSTRACT

Pig iron in the tap spout of a blast furnace is desulfurized by passing it through a vessel containing a rotating inverted Tshaped stirring element which mixes the pig iron and a desulfurizing agent as they pass from the inlet to the outlet of the vessel. From the outlet, a trough conveys the treated pig iron to a flue with an overflow edge located at a level above the bottom surface of the outlet. The flue is arranged to separate the pig iron from the spent desulfurizing agent.

United States Patent Schulz et al.

[451 Aug. 29, 1972 [54] METHOD OF AND APPARATUS FOR DESULFURIZING PIG IRON [72] Inventors: Hans-Peter Schulz, Hattingen; .Iorn Mandel, Gelsenkirchen, both of Germany [73] Assignee: Rheinstahl Huttenwerke A.G., Am

Pheinstahlhaus, Germany [22] Filed: Sept. 4, 1970 21 App1.No.: 69,600

[30] Foreign Application Priority Data Sept. 6, 1969 Germany ..P 19 45 275.6

[52] US. Cl. ..266/34 A, 75/58, 266/34 T [51] Int. Cl ..C2lc 7/00 [58] Field of Search ..75/45, 46, 53-58,

75/61', 266/34 R, 34 A, 34 T, 37-40 [56] References Cited UNITED STATES PATEN'IS 1,590,730 Evans ..266/38 2,290,961 7/ 1942 l-Ieuer ..266/34 R 2,319,402 5/ 1943 Heuer ..266/37 2,677,609 5/1954 Moore et al. ..75/53 3,572,671 3/1971 Worner ..266/39 FOREIGN PATENTS OR APPLICATIONS 154,998 9/1962 U.S.S.R. ..266/37 Primary Examiner-Gerald A. Dost AttorneyMcGlew and Toren [57] ABSTRACT Pig iron in the tap spout of a blast furnace is desulfurized by passing it through a vessel containing a rotating inverted T-shaped stirring element which mixes the pig iron and a desulfurizing agent as they pass from the inlet to the outlet of the vessel. From the outlet, a trough conveys the treated pig iron to a flue with an overflow edge located at a level above the bottom surface of the outlet. The flue is arranged to separate the pig iron from the spent desulfurizing agent.

11 Claims, 3 Drawing Figures Patented Aug. 29, 1972 3,681,430

.2 Sheets-Sheet 1 AWEWH 0 I 2 I I /NVENTOR HANS P ER scu JORN M DEL v ATTORNEYS Patented Au'g.29, 1972 3,687,430

2 Sheets-Sheet 2 INVENTORS HANS- PETER SCHULZ JORN MANUEL ATTORNEYS METHOD OF AND APPARATUS FOR DESULFURIZING PIG IRON SUMMARY OF THE INVENTION The present invention is directed to an apparatus for and a method of desulfurizing pig iron and, more particularly, it is directed to an arrangement for stirring the pig iron and a desulfurizing agent as it passes through a vessel in the tap spout of a blast furnace.

It is known to transfer over 90 percent of the sulfur used in blast furnaces into the slag by utilizing a highly basic slag. Such a desulfurization process is less expensive than desulfurization in a steel mill or foundry. However, it has a number of disadvantages. Initially, in such a process the specific coke consumption is increased and the output decreases. Secondly, the alkali oxides introduced over the blast furnace burden and the coke are absorbed only to a minor degree by the basic slag in the blast furnace and, as a result, they accumulate in the blast furnace and lead to increased incrustation and wear of the brickwork which, in turn, results in difficulties in the use of the furnace.

To overcome these disadvantages, several methods have been developed for desulfurizing pig iron in ladles outside the blast furnace, in one such development a stirring apparatus (See German Utility Patent, 988,244) which has provided good desulfurizing results, but such processes are disadvantageous at least in that the desulfurization takes place as an additional stage and disturbs the flow of the material. Further, it delays the circulation of the ladles, and also requires additional switching and crane operations. A particular source of difficulty with this arrangement is the removal of the slag from the ladles after the desulfurization treatment. At the present time a clean slagoff with low iron losses is not possible by means of mechanical devices and requires hand operation. Therefore, ladle desulfurization methods have been used only in a few special cases, for example, in smelting silica-rich iron ores to reduce the specific amount of slag to economical values, and in the production of low-sulfur special steels or for reducing excessive sulfur contents to avoid problems in the operation of the furnace.

To achieve desulfurization of pig iron without impairing the operation of the blast furnace and without causing any problems in the flow of the material, it has been attempted to carry out the desulfurization in the tap spout. In such an arrangement the desulfurization slag within the spout can be retained by a flue or siphon so that manual slag removal is eliminated. However, when such a method is used as compared to desulfurization in the ladle, the reaction time is predetermined by the greatly varying pig iron tapping rate of the blast furnace. To reduce the effect of the tapping rate, the desulfurization has been effected over a longer path of the tapping spout (Stahl und Eisen, 1969, p. 776 to 780). in mixing the lighter desulfurizing agent with the pig iron, devices with porous bottoms, such as rotating drums and trough sections, have been used for introducing gas through the porous bottoms (Adrianov, V.F. Sikaovyj rezim domennych pecey, Moscow 1967, p. 295-203 and Wosko Boinikoff, W. G. Stal in Deutsch, 1955, no. 7). These proposed desulfurization processes have not been successful in practice because it has not. been possible to control completely the mixing of the desulfurizing agents with the pig iron and, further, satisfactory desulfurization results can only be achieved with known continuous desulfurization methods operating at low tapping rates of 1 to 3 t/min (tons per minute) and only with liquid desulfurizing agents, such as soda. In these known processes the apparatus expenditure is high and its safety is low such as with the known rotating drum which presents difficulties in replacement if any problem arises. Additionally, the quantity of agents required, even for low degrees of desulfurization, is high.

Therefore, in view of the present state of the technology, the primary object of the present invention is to afford a safe method and apparatus for desulfurizing pig iron in a blast furnace tap spout by adding a desulfurizing agent which permits a uniform and considerable reduction in the sulfur content, even with a greatly varying tapping rate of the pig iron because of a particularly high reaction velocity. Further, the arrangement is also suitable for processing the entire pig iron production of a large blast furnace and it is not limited to the use of soda as a desulfurizing agent.

Therefore, in accordance with the present invention, a vessel having a refractory lining is located in the tap spout of a blast furnace and contains an inlet and outlet to provide through flow of the pig iron. The bottom surface of the outlet is substantially at the level of the bottom surface of the vessel. From its outlet, a trough conveys the flow of pig iron to a flue with an overflow edge located at a level above the bottom surface of the outlet. Within the vessel a known stirring element of inverted T-shaped construction is arranged to rotate about its vertical axis with its transverse head or blade rotating in the path of flow of the pig iron between the inlet and the outlet. Further, the desulfurizing agent is supplied into the vessel so that it is mixed with the pig iron by means of the stirring member.

The intensive mixing of the desulfurizing agent and the pig iron flowing through the vessel achieved by means of the stirring member produces such a high reaction velocity that the desulfurization is completed to the desired degree even where variations in the pig iron flow occur due to frequent variations in the tapping rate.

In view of the known continuous desulfurization methods, the results provided by the present invention are most surprising. In the known continuous desulfurization processes, for example, in rotating drums, a desulfurizing agent is used which forms a liquid slag,

that is soda, the rotation of the drum effects a thorough Y mixing of the desulfurizing agent with the pig iron melt and the stay period of the unit volumes of pig iron and slag passing through the rotating drum are the same for a certain rate of flow of the pig iron. However, in spite of these apparently optimum desulfurization conditions in known rotating drums, the extent of desulfurization is unsatisfactory at high tapping rates. Therefore, it.

could not be expected that the moderate desulfurization results of the known rotating drum would be achieved, not to mention exceeded, with the apparatus according to the present invention where a unit volume of pig iron is discharged from the vessel after a single revolution of the stirring member, or after several revolutions so that there is an appreciable range for the dwell period for individual unit volumes of the pig iron and slag. it would be considered that the range of the dwell period prevents the achievement of a total low sulfur content since the unit volumes only briefly desulfurized would be fed with a relatively high sulfur content from the apparatus and the unit volumes which are desulfurized'for a much longer time would have a relatively much lower sulfur content and the desulfurization rate would be hardly more effective than with a medium dwell period because of the approximation with equilibrium values. Consequently, it would be expected that the unit volumes desulfurized for a longer period of time would afford no compensation for those unit volumes which were only briefly desulfurized. It has been found, however, that good desulfurization results of the total pig iron melt with low sulfur contents of about 0.005 percent can be achieved in accordance with the present invention, despite the wide variation in the range of the dwell period. Additionally, these results are attained not only with the use of soda as the desulfurizing agent, which forms a liquid slag, but also with solid desulfurizing agents, such as lime and calcium carbide. This characteristic of the invention represents a considerable technical advance, since soda must be employed with great caution as a desulfurizing agent because of the harmful vapors generated, furthermore, the slag formed in desulfurization with soda cannot be stored or otherwise used without complicated precautions to prevent contamination or pollution of ground water. The reliability of the present invention in achieving low end sulfur contents, and the simple rugged construction provided by its apparatus, which is not susceptible to operating difficulties, permits the desulfurization of the entire production of large blast furnaces. An important characteristic of this arrangement, is the ability of the apparatus to be removed and replaced by a spare unit without further adaption.

Further, the method of the present invention can be carried out to effect a very thorough mixing of the pig iron and desulfurizing agent, and thereby attain a rapid desulfurization, by using a stirring member having an inverted T-shaped form with the bottom beam-shaped portion of the stirring element sheathed in refractory material. In many blast furnace plants there is no great vertical difference between the level of the tap hole and the top edge of the transfer ladles which receive the pig iron. In the present invention it is recommended that the bottom surface of the inlet for flowing pig iron into the vessel is located at the level of the bottom inner surface of the vessel, and that the top surface of the inlet is lower than the overflow edge of the flue while the top surface of the outlet is slightly higher than the overflow edge. The vessel contains an opening for adding the desulfurizing agent for mixture with the pig iron. The level of the pig iron within the vessel is kept constant by the overflow edge of the flue. Accordingly, the top surface of the outlet retains the desulfurizing agent so that it exits from the vessel at a somewhat lower rate than the pig iron. This feature enhances the effective utilization of the desulfurizing agent. The inner surface of the inlet due to its relative position to the overflow edge mentioned above, is lower than the constant pig iron level in the vessel so that the ejection of desulfurizing agents by the stirring member as well as the introduction of blast furnace slag into the vessel are avoided.

If there is a considerable vertical difference between the tap hole of the blast furnace and the upper edge of the transfer ladle, it is recommended that the inlet for the pig iron into the vessel is arranged in its cover and the top surface of the outlet is located slightly higher than the overflow edge of the flue for the pig iron. In this arrangement of the apparatus, the kinetic energy of the pig iron jet falling freely into the pig iron bath in the vessel is also used for mixing the pig iron and the desulfurizing agent. Also, the desulfurizing agent is added into the pig iron jet as it falls into the vessel. Since the bottom of the vessel and the bottom of the outlet form a continuous smooth surface and the bottom of the vessel and the outlet are slightly inclined downwardly in the direction from the inlet to the outlet, and also since the outlet widens as it opens outwardly from the vessel, a simple smooth vessel form is provided from which the pig iron and slag flow off almost completely at the end of the tapping operation whereby the vessel need not be cleaned before it is reused. Because of the widening of the outlet, deposits at that location are avoided, such as are frequently observed with the addition of nonmelting desulfurizing agents, such as lime and calcium carbide. In order to prevent any short circuiting about the stirring element between the inlet and the outlet, the bottom edge of the beam-shaped stirring element is arranged between 2 and 10 cm above the bottom of the vessel, as under 2 cm, the danger of striking against slag residue is too great, and above 10 cm there is a risk that short circuiting might occur.

When large amounts of pig iron are passing through the vessel, for example, at the rate of 6 t/min., a good desulfurization is achieved in a vessel whose inside diameter is greater than 1.5 m and where the length of the beam-shaped stirring element is 0.35 to 0.45 times the inside diameter of the vessel.

With these dimensions of the stirring member, a vigorous and rapid mixing of the desulfurizing agent and the pig iron is achieved with a resultant high reaction velocity. With lower tapping rates of up to 3 t/min., the same result is attained if the interior diameter of the vessel is less than 1.5 m and the length of the beamshaped stirring element is 0.45 to 0.80 times the inside diameter of the vessel. The speed of revolution of the stirring member is advantageously between and rpm. A long contact period between the pig iron and the additive can be achieved if the inlet for the pig iron is aligned in the direction of the axis of the vessel and if the outlet for the pig iron is arranged in the third quadrant of the vessel, counting the quadrants in the direction of rotation starting from the inlet in the direction of rotation of the stirring member, and with the opening for the addition of the desulfurizing agent located in the fourth quadrant. This arrangement of flow through the vessel counteracts any direct passage by encouraging the formation of circular currents within the vessel. Since the inlet for the desulfurizing agent is located in the fourth quadrant, initially the additive is moved by the stirring element, away from the outlet, and is mixed with the pig iron in the range of the inlet by the turbulence with the entering flow of pig iron. To prevent the ejection of pig iron above the edge of the vessel, its upper edge is arranged at least 60 cm above the upper edge of the pig iron outlet. For pig iron tapping rates of l t/min, vessel diameters of 1.2 m are advisable, for tapping rates of 6 t/min the vessel diameter should be about 2 m. By lowering the degree of basicity of the blast furnace slag the output of the blast furnace can be substantially increased, as is known. Further the operation of the blast furnace becomes more uniform and the danger of suspensions is greatly reduced. This particular method of increasing the output of blast furnaces could not be used up until the present time because the resulting high sulfur contents required desulfurization in special apparatus and in the steel mill which was too expensive. It has been found, however, that even high initial sulfur contents of 0.070 percent can be reduced with surprising safety and reliability to sulfur contents under 0.010 percent with the desulfurizing apparatus in accordance with the present invention. This apparatus also permits the use of coke having a sulfur content of over 1 percent, though it had not been possible to use such coke in the past. Accordingly, the invention provides that with a blast furnace slag with a base degree of CaO/SiO which is less than 1.2, even when using coke with a high-sulfur content of over 1 percent, it is possible to reduce the sulfur content to lower sulfur values.

Sulfur contents of less than 0.010 percent can be reliably achieved with the present method disclosed herein and the furnace output is increased by to percent compared to conventional methods; additional cost savings are possible by utilizing cheap high-sulfur coke.

The various features of novelty which characterize the invention are pointed out with particularity in the claims annexted to and forming a part of this specification. For a better understanding of the invention, its operating advantages and specific objects attained by its uses, reference should be had to the accompanying drawing and descriptive matter in which there is illustrated preferred embodiments of the invention.

BRIEF DESCRIPTION OF THE DRAWING In the drawing:

FIG. I is a vertical sectional view of apparatus for desulfurizing pig iron, in accordance with the present invention, with a horizontal inlet for the pig iron into a vessel forming a part of the apparatus;

FIG. 2 is a vertical sectional view, similar to FIG. 1, illustrating another embodiment of the present invention with a pig iron inlet located in the upper end of the vessel; and

FIG. 3 is a horizontal sectional view through the vessel shown in FIG. 1.

DETAILED DESCRIPTION OF THE INVENTION In FIG. 1, apparatus for desulfurizing pig iron in the tap spout of a blast furnace is disclosed in which a desulfurizing agent 1 is supplied into a vessel 2 through an opening 12 located in the upper surface of the vessel and pig iron is admitted into the vessel through an inlet 3 located at the bottom of the vessel and an outlet 4 for the treated pig iron is located on the bottom of the vessel on the opposite side from the inlet. A trough extends from the outlet 4 to a flue 6 having an overflow edge 7 for the treated pig iron. Supported on top of the vessel 2 is a stirring member 8 which rotates about its vertical axis and has a vertically depending shaft 8a and a horizontally disposed beam-shaped stirring part 9 attached to the lower end of the shaft and forming, in combination therewith, an inverted T-shaped structure. The stirring part 9 is covered with refractory material and a gear motor 10 is mounted on the upper surface of the vessel 2 for rotating the shaft 8a and stirring part 9 about the vertical axis. As illustrated in FIG. 1 the pig iron inlet 3 and outlet 4 are located at about the same level and the lower surface of each is flush with the bot tom interior surface 5 of the vessel 2. The bottom surface 5 of the vessel and the outlet 4 are slightly inclined in the direction of flow to facilitate the emptying of the vessel after the tapping operation is completed.

In FIG. 3, the direction of rotation of the stirring member 8 is shown by the arcuate arrows and considering the inlet 3 as located on the radius between the first and fourth quadrants, counting the quadrants in the direction of rotation of the stirring member, the outlet 4 is located in the third quadrant. Further, as shown in FIG. 3, the outlet 4 conically widens or diverges toward the outside to a trough 20 to avoid any scaffolding of the slag. The bottom surface 11 of the beam-shaped stirring part 9 of the stirring member is positioned closely above the bottom surface 5 of the vessel, that is, about 5 cm. With this arrangement any short circuiting of the pig iron between the inlet and outlet without reaction with the desulfurizing agent is avoided. As indicated in FIG. 3 the pig iron inlet 3 is located on one of the diameters of the vessel 2. The desulfurizing agents 1 are added through the opening 12 in the top of the vessel into the pig iron entering through the inlet and the opening 12 is located in the fourth quadrant of the vessel based on the direction of rotation of the stirring member. Accordingly, the arrangement of the pig iron inlet and the opening for the desulfurizing agents assure an increased dwell period of the desulfurizing agent within the vessel and, accordingly, in contact with the pig iron. The length of the stirring part 9 is 0.5 times the inside diameter of the vessel, and the upper edge of the vessel is positioned cm above the upper edge or surface of the outlet 4 to prevent any spurt out of the pig iron during stirring. The height or level of the pig iron within the desulfurizing vessel is determined by the position of the overflow edge 7 of the flue 6 located in the trough 20 spaced outwardly from the outlet 4, and the arrangement of the flue affords the separation of spent desulfurizing agent from the pig iron.

In another embodiment of the apparatus, according to the present invention, an inlet 3a for the pig iron is located in the upper surface or cover 13 of the vessel 2. As indicated in FIG. 2, the pig iron and the desulfurizing agent enter the vessel through the same inlet 3a. As shown in FIG. 3, an emergency tap 14 is located in the vessel for emptying it in the event any difficulties arise. After the tapping of the blast furnace is completed, the flue is pierced in its lower portion at or shortly before the end of the tap so that the solid desulfurization slag is retained or is bypassed.

The desulfurizing apparatus as illustrated in FIG. 1, has been used to desulfurize open hearth pig iron in blast furnace tapping.

The following table displays the results for high pig iron tapping rates of 4 7 t of pig iron/min with a vessel of 2,000 mm inside diameter and also for low tapping rates of about 1 t of pig iron/min with a smaller vessel of 1,200 mm inside diameter. Different agents were employed for achieving the desulfurization. When lime and methane were used, the methane was injected into the melt through a blow lance located outside the range of the beam-shaped stirring part 9.

4. Apparatus for desulfurizing pig iron, as set forth in claim 1, wherein the upper surface of the outlet from said vessel is located slightly above the level of the overflow edge of said flue, and the inlet for the pig iron Table: Continuous Desullurization of Open Hearth Pig Iron Length of Beam-Shaped Part of Stirring Element:

800 mm. Speed of Rotation of Stirring Element: 74 r.p.m.

Sulfur Chemical analysis of pig iron content IlgSidt; diami lIean pig irgn Desullurizing agent before treatment (percent) treat e er vesse apping la e (m.m.) (t.p.i./min.) Type (1 /t.p.1.) C Si Mn P S (percent) 2,000 6. 8 {85% CaCz kg 4. 42 0.67 1. 60 0.080 0. 018 0. 006

CaCOa. 2,000 4. 5 {85% CaCz E. 4 kg. 4. 45 0.55 1. 55 0. 091 0. 029 0. 005

+15% CaCOa- 2,000 5.7 CaC2 5.4 kg"... 4.48 0.57 1.52 0.090 0.022 0.006 1000 4 0. 54 1. 62 0. 095 o. 032 0.011 1,200 0 9 5. 4 kg 0.62 1. 70 0. 002 0. 023 0. 006 1,200 1 1 d0 9.5kg 4.49 0.63 1. 58 0.083 0.017 0.005

The apparatus described above can also be used for alloying and for the primary purification of pig iron or other liquid metal melts during tapping. The apparatus can also be used with advantage for the desulfurization of cast iron in foundries. With the usual outputs of cupola furnaces used in such operations of about 1 ton per hour, the vessel can be small and can be easily connected into the usual flow of the material.

What is claimed is:

1. Apparatus for desulfurizing pig iron in the tap spout of a blast furnace with the addition of a desulfurizing agent, comprising a vessel lined with refractory material and having an inlet thereto and an outlet therefrom with the outlet spaced angularly from the inlet, the lower surface of the outlet is located in the plane of the interior lower surface of said vessel, a trough is connected at one end to said outlet from said vessel and extends outwardly therefrom, a flue positioned in said trough spaced from the outlet and having an overflow edge spaced upwardly from the lower surface of the outlet from said vessel, means for introducing desulfurizing agent into the pig iron, a stirring member positioned within said vessel between its inlet and outlet, said stirring member comprising a motor mounted on the upper portion of said vessel and arranged to rotate said stirring member about its vertical axis and to effect the mixture of the pig iron and the desulfurizing agent.

2. Apparatus for desulfurizing pig iron, as set forth in claim 1, wherein said stirring member has a vertically depending shaft part, and a horizontally arranged beam-shaped stirring part attached to the lower end of said shaft part and combining therewith to form an inverted T-shaped structure, the stirring part 9 is sheathed in refractory material.

3. Apparatus for desulfurizing pig iron, as set forth in claim 2 wherein the bottom surface of said inlet for pig iron is located substantially at the level of the bottom interior surface of said vessel, the upper surface of said inlet is located below the level of the overflow edge and the upper surface of the outlet is located slightly above the level of the overflow edge of said flue for the pig iron, and an opening for supplying desulfurizing agent into the vessel is located in the upper surface thereof above the level of the pig iron.

is arranged in the upper surface of the vessel above the upper surface of the outlet from said vessel.

5. Apparatus for desulfurizing pig iron, as set forth in claim 1, wherein the interior bottom surface of said vessel and the lower surface of the outlet form a smooth continuous surface and the interior bottom of the vessel and the lower surface of the outlet are slightly inclined downwardly in the direction from the inlet to the outlet, and the opposed surfaces of the outlet as they proceed outwardly from the interior of said vessel are in diverging relationship.

6. Apparatus for desulfurizing pig iron, as set forth in claim 2, wherein the lower surface of said beam-shaped stirring part of said stirring member is positioned closely above the interior bottom surface of the vessel in the range of about 2-10 cm above the interior bottom surface.

7. Apparatus for desulfurizing pig iron, as set forth in claim 2, wherein with said vessel having an inside diameter of more than 1.5 m, the length of said beamshaped stirring part is 0.35 to 0.45 times the inside diameter of said vessel.

8. Apparatus for desulfurizing pig iron, as set forth in claim 2, wherein with said vessel having an inside diameter of less than 1.5 m, the length of the beamshaped stirring part is 0.45 to 0.80 times the inside diameter of said vessel.

9. Apparatus for desulfurizing pig iron, as set forth in claim 1, wherein the inlet to said vessel is located on one of the axes of said vessel and said vessel is divided into a first quadrant, a second quadrant, a third quadrant, and a fourth quadrant counting from said inlet in the direction of rotation of said stirring member, said outlet is spaced angularly from said inlet and is located in the third quadrant of said vessel, and the opening for supplying desulfurizing agent into said vessel is located in its fourth quadrant.

10. Apparatus for desulfurizing pig iron, as set forth in claim 1, wherein the upper edge of said vessel is located at least 60 cm above the upper surface of the outlet therefrom.

11. Apparatus, as set forth in claim 4, wherein said desulfurizing agent is supplied into said vessel through the inlet therein for said pig iron. 

2. Apparatus for desulfurizing pig iron, as set forth in claim 1, whErein said stirring member has a vertically depending shaft part, and a horizontally arranged beam-shaped stirring part attached to the lower end of said shaft part and combining therewith to form an inverted T-shaped structure, the stirring part 9 is sheathed in refractory material.
 3. Apparatus for desulfurizing pig iron, as set forth in claim 2, wherein the bottom surface of said inlet for pig iron is located substantially at the level of the bottom interior surface of said vessel, the upper surface of said inlet is located below the level of the overflow edge and the upper surface of the outlet is located slightly above the level of the overflow edge of said flue for the pig iron, and an opening for supplying desulfurizing agent into the vessel is located in the upper surface thereof above the level of the pig iron.
 4. Apparatus for desulfurizing pig iron, as set forth in claim 1, wherein the upper surface of the outlet from said vessel is located slightly above the level of the overflow edge of said flue, and the inlet for the pig iron is arranged in the upper surface of the vessel above the upper surface of the outlet from said vessel.
 5. Apparatus for desulfurizing pig iron, as set forth in claim 1, wherein the interior bottom surface of said vessel and the lower surface of the outlet form a smooth continuous surface and the interior bottom of the vessel and the lower surface of the outlet are slightly inclined downwardly in the direction from the inlet to the outlet, and the opposed surfaces of the outlet as they proceed outwardly from the interior of said vessel are in diverging relationship.
 6. Apparatus for desulfurizing pig iron, as set forth in claim 2, wherein the lower surface of said beam-shaped stirring part of said stirring member is positioned closely above the interior bottom surface of the vessel in the range of about 2-10 cm above the interior bottom surface.
 7. Apparatus for desulfurizing pig iron, as set forth in claim 2, wherein with said vessel having an inside diameter of more than 1.5 m, the length of said beam-shaped stirring part is 0.35 to 0.45 times the inside diameter of said vessel.
 8. Apparatus for desulfurizing pig iron, as set forth in claim 2, wherein with said vessel having an inside diameter of less than 1.5 m, the length of the beam-shaped stirring part is 0.45 to 0.80 times the inside diameter of said vessel.
 9. Apparatus for desulfurizing pig iron, as set forth in claim 1, wherein the inlet to said vessel is located on one of the axes of said vessel and said vessel is divided into a first quadrant, a second quadrant, a third quadrant, and a fourth quadrant counting from said inlet in the direction of rotation of said stirring member, said outlet is spaced angularly from said inlet and is located in the third quadrant of said vessel, and the opening for supplying desulfurizing agent into said vessel is located in its fourth quadrant.
 10. Apparatus for desulfurizing pig iron, as set forth in claim 1, wherein the upper edge of said vessel is located at least 60 cm above the upper surface of the outlet therefrom.
 11. Apparatus, as set forth in claim 4, wherein said desulfurizing agent is supplied into said vessel through the inlet therein for said pig iron. 